This invention relates to an apparatus for coating or applying a sterilizing agent such as hydrogen peroxide to a web, usually a continuous web, the invention exhibiting particular utility in the field of aseptic packaging.
Aseptic packaging may be defined as the packaging of foodstuffs which have been sterilized such as milk and fruit juices as well as solid foods, in containers which are themselves sterile. The sterility of the completed container helps insure that spoilage of the packaged foodstuff will not occur, or will occur only after a long period of time, even in the absence of refrigeration. Typically, an aseptic package is formed by passing a continuous web of a plastics coated paperboard material through a forming apparatus, the forming apparatus folding the web into the form of a container. The container is then filled with the foodstuff, cut from the remainder of the web and sealed. In addition to the requirement that this apparatus operate in a sterile atmosphere (sometimes termed sterile ambient) it is necessary that the web be coated with a sterilizing agent, such as hydrogen peroxide, on at least that surface which will define the interior surface of the containers. Such a sterilizing agent is necessary because, from the time of its manufacture to the time of its ultimate use in making aseptic packages, the web may be exposed to a variety of contaminating agents or substances. Accordingly, it is desirable that the web be sterilized immediately prior to its formation into packages, U.S. Pat. No. 3,947,249 issued on its face to Egger hereby incorporated by reference, illustrates an apparatus for packaging under sterile conditions.
The type of sterilizing agent and the amount of it applied to the web have been more or less codified into industry standards in the field of aseptic packaging. These standards have been set, in the United States, by the Food and Drug Administration (FDA). For example, for a one quarter liter aseptic package, industry standards require that a minimum of 89 to a maximum of 133 ml. of hydrogen peroxide be coated on one web surface per 1,000 packages.
The art of coating a web embraces a great variety of coating apparatus, including the use of coating rollers between which a material to be coated, such as a web of indefinite length, passes. Typically, the rollers are formed in pairs and rotate on horizontal axes, one roller being above the other with the lower (application) roller being partially immersed in a bath or vat of a coating material. The present invention employs rollers of this type, with the lower roller being partially immersed in a bath of hydrogen peroxide.
In addition to the FDA regulations regarding the amounts of sterilizing agent applied, it is necessary that the coating be uniformly applied. While simple in concept, the practical application of such rolls for coating a web of plastics material for aseptic packaging reveals that there exists parameters associated with the coating rollers which must be observed if the desired uniformity of the coating is to be obtained.
In the coating of the web material with hydrogen peroxide, it is usual that different stock rolls of the web material will contain different amounts of moisture in the paperboard, for example. This, together with changes in ambient temperature and humidity, yields the undesirable result that upon changing web stock rolls (as they become used up), the nip pressure setting which yielded a desired peroxide coating rate for the previous (used up) web roll will yield a sufficient peroxide coating rate for the new roll of web material. Accordingly it is necessary, in some cases, to reset the nip pressure with each new stock web roll.
With a smooth-surfaced applicator roll or with an applicator roll with discrete depressions on its surface, it was not possible, to obtain the desired FDA coating rates or minimum and maximum range no matter what the nip pressure was.